"Autofill" is a thing that exists separate to fill, which is enabled when projected queue times are too long. For big regions (e.g. NA) at most hours of the day at most elos, this isn't something that actually comes into play. However, for some elos (usually extremely high or extremely low) or at quiet hours of the day, the projected queue times can be quite long, and so autofill is enbabled, which means anyone without autofill protection can be given any role, not just one of the two they selected. This results in somewhat faster games and prevents people from potentially being in queue for literal hours, but can be somewhat unpleasant for those who get a role they haven't practiced in some time.
edit: You are warned when autofill is enabled though, so people always know in advance that it's a thing. If you've never had the warning come up, it's likely autofill isn't something likely to come up for your particular role preferences/elo/time of day, and so you would be guaranteed one of your two selected roles regardless.